The molecular interactions of natural immunoglobulin (Ig) M antibodies and components of the alternative complement pathway in opsonization and phagocytosis of Bacteroides fragilis and Bacteroides thetaiotaomicron will be investigated. Normal human IgM and human C3, B, D, P, H, and I will be purified to homogeneity by chromatographic methods. Bacteria will be incubated with mixtures of the alternative pathway proteins and tracelabeled C3, and the kinetics of deposition of C3 fragments on the bacterial surfaces will be measured. The ability of surface-bound natural IgM antibodies to augment the rate or magnitude of binding of C3 fragments and B from the fluid phase onto the bacterial surfaces during alternative pathway activation will be determined. Binding of IgM to the bacterial surfaces will also be quantitated. If augmentation by IgM is demonstrated, experiments will be performed to investigate the mechanism of augmentation. In addition, the ability of C3 fragments deposited on the bacterial surfaces after assembly of alternative pathway convertases to facilitate uptake and intracellular killing of labeled bacteria by human polymorphonuclear leukocytes in the absence of immunoglobulin will be determined, and the relative roles of C3b and iC3b in facilitating these events will be investigated. The augmentative effect of surface-bound IgM on C3 fragment-mediated uptake and intracellular killing will also be assessed. Ingestion will be confirmed by electron microscopy. Rabbit IgM and IgG antibodies directed against the capsular polysaccharide and lipopolysaccharide moieties of the outer membrane complex of B. fragilis will be purified by affinity chromatography. The ability of the antibodies to facilitate uptake by human polymorphonuclear leukocytes of the homologous strain and heterologous strains of B. fragilis will be compared. The fundamental knowledge derived from this investigation will increase understanding of natural and acquired host resistance against Bacteroides and may lead to the development of immunological approaches for the prevention, diagnosis, or therapy of infections caused by these anaerobic bacteria.